Definition
A valve in an aircraft fuel or hydraulic system that allows fluid to flow between two parallel lines, tanks, or system sides. In fuel systems, it permits fuel to move from one wing tank to the other to balance lateral weight or feed an engine on the opposite side. In hydraulic systems, it can connect parallel circuits so fluid can pass between them when needed.
Plain English
A valve that lets fluid pass sideways between two separate lines or tanks, instead of just flowing straight through one path.
Context Anchor
Seen in fuel system and airframe maintenance discussions, especially on aircraft with tanks or fuel cells in both wings.
Derivation
Cross means across or sideways, and flow refers to the movement of fluid. So the name simply describes a valve that lets fluid flow across from one side of a system to the other.
Why Pilots Care
Keeps the aircraft laterally balanced and allows continued flight if one tank runs low, directly affecting safety and range.
Intuition Check
Do not read “cross-flow” as airflow moving sideways over the airplane. Here it means fluid moving across from one side of the aircraft to the other through a controlled valve.
Example Sentence 1
The technician opened the cross-flow valve to allow fuel to transfer from the left wing tank to the right wing tank.
Example Sentence 2
Opening the cross-flow valve allowed fuel to move from the fuller tank to the lower one, restoring balance.